The most accurate current method to measure body composition is water immersion densitometry. Water immersion densitometry is based on Archimedes' Principle, which states that the buoyancy force on an object submerged in water equals the weight of the water the object displaced. By determining the mass of an object dry, and then in water, the difference, divided by the density of the water, is the volume of the object. Once volume is determined by water immersion, that person's "residual" lung volume must be entered into the equation. The residual lung volume is the volume of the air still in the lungs of the submerged person, and must be known to accurately determine body composition. With the total volume of the person known, weight is measured, and from an equation describing human body composition based on density, the body fat content and lean body mass are calculated.
The drawbacks of water immersion densitometry are that it is a lengthy and uncomfortable process. It requires the person to be at ease in the water, and be able to remain underwater after exhaling all the air from his or her lungs. The residual lung volume test is also lengthy and uncomfortable. The equipment to perform this test is expensive and has a great maintenance cost associated with it. The reported accuracy of water immersion densitometry is .+-.2%. Air immersion has been suggested, but the cylinders and pistons and the associated equipment necessary to operate these devices make them unattractive also.
The cheapest and most popular method of determining body fat content is the "Calipers" method. In this test the person's skin is pinched in several places and the thickness of the layer of skin and subcutaneous fat is measured with a caliper. From a look-up table based on sex and age, one's body fat content is determined. This method is cheap, comfortable and easy. However, the accuracy is only reported to be .+-.10%, and the repeatability is questionable.
Another method is based on bioelectric impedance measurements. In this method an electrical current is passed through the body and the body's reactances are measured, for reactance is influenced by the amount of subcutaneous fat. A computer uses this result to calculate body fat content. The reported accuracy is less than water immersion and is heavily affected by body water retention at the time of the test.
Yet another method is based on infrared measuring, where an infrared beam is directed into the skin on a person's writing arm biceps. The muscle tissue reflects the infrared beam allowing the thickness of the subcutaneous fat to be measured in that one spot. A computer then determines body fat content based on this fat thickness sample and some general fitness questions asked of the person being measured. This method is not considered to be very accurate.
There are other methods of determining body composition including photon absorption, tomography, magnetic resonance, and dissection, which are even less appealing due to complexity, cost and/or difficulty.